1. Field of the Invention
The invention relates to burners and, more particularly, relates to gas fired burners usable to heat and dry materials in rotary drums commonly used in the production of hot mix asphalt (HMA) or in soil remediation.
2. Background of the Invention
Gas fired burners are nearly universally used to supply heat to rotary dryers of the type commonly used for the production of HMA or for soil remediation. Burners employed in such plants, commonly known as nozzle mix burners, typically supply air and a gaseous fuel to the combustion chamber of the dryer via separate inlets essentially resulting in simultaneous mixing and combustion. Combustion air for such burners is supplied directly by a primary blower and indirectly by a secondary blower or by convection. This technique leads to inefficient mixing and thus incomplete combustion of the fuel, resulting in the emission of increased levels of hydrocarbons and other volatile organic compounds (VOCs). VOC emissions from such burners will soon render their use unacceptable in many areas such as in the southern portion of California where ever stricter emission standards are being imposed.
VOC emissions from conventional nozzle mix burners can be reduced by supplying high amounts of excess air to the combustion chamber of the dryer (with excess air being defined as the amount in excess of that required for stoichiometric combustion), thereby promoting combustion of a higher percentage of fuel. Standard nozzle mix burners currently employed in most HMA plants require from 50% to 200% excess air.
The use of large amounts of excess air to reduce VOC emissions exhibits at least two drawbacks each of which could independently render standard nozzle mix burners commercially unacceptable in the near future. First, using large amounts of excess air significantly increases capital expenditure and production costs because larger and higher powered blowers are required to force the excess air through the system and because this excess air must be heated to maintain acceptable operating temperatures in the dryer, thereby requiring the consumption of more fuel. Second, combustion in the presence of excess air leads to increased NOx emissions levels because there is more free oxygen available to combine with nitrogen. Because NOx emissions are also heavily regulated, a plant which uses excess air in its burner to reduce VOC emissions may still fail to meet environmental regulations because of unacceptably high NOx emissions.
It is known in other industries to use premix burners to reduce VOC and NOx emissions by premixing the fuel and air prior to combustion, thereby reducing VOC emissions without requiring high amounts of excess air and thereby reducing NOx emissions. Such premix burners have, however, never gained acceptance in HMA or soil remediation plants, possibly because they employ a separate mixing device located upstream of the burner which substantially increases the cost and size of the burner assembly.